Many devices used in biomedical applications require that the bulk of the device have one property and the surface of the device have a different property. For example, contact lenses may require relatively high oxygen permeability through the bulk of the lens to maintain good corneal health. However, materials that exhibit exceptionally high oxygen permeability (e.g. polysiloxanes) are typically hydrophobic and, untreated or not surface modified, will adhere to the eye. Thus, a contact lens will generally have a core or bulk material that is highly oxygen permeable and hydrophobic, and a surface that has been treated or coated to increase hydrophilic properties. This hydrophillic surface allows the lens to move relatively freely on the eye without adhering excessive amounts of tear lipid and protein.
A known method for modifying the hydrophilicity of a relatively hydrophobic contact lens material is through the use of a plasma treatment. Plasma treatment techniques are disclosed, for example, PCT Publication Nos. WO 96/31793 to Nicholson et al., WO 99/57581 to Chabrececk et al., and WO 94/06485 to Chatelier et al. In the Chabrececk et al. application, photoinitiator molecules are covalently bound to the surface of the article after the article has been subjected to a plasma treatment which provides the surface with functional groups. A layer of polymerizable macromonomer is then coated onto the modified surface and heat or radiation is applied to graft polymerize the macromonomer to form the hydrophilic surface.
Plasma treatment processes, however, require a significant capital investment in plasma processing equipment. Moreover, plasma treatments take place in a vacuum and, thus, require that the substrate be mostly dry before exposure to the plasma. Thus, substrates, such as contact lenses, that are wet from prior hydration or extraction processes must be dried, thereby further adding to both the capital and production costs. As a result of the conditions necessary for plasma treatment, the incorporation of a plasma treatment process into an automated production process is extremely difficult.
Other methods of permanently altering the surface properties of polymeric biomaterials, such as contact lenses, have been developed. Some of these techniques include Langmuir-Blodgett deposition, controlled spin casting, chemisorptions, and vapor deposition. Examples of Langmuir-Blodgett layer systems are disclosed in U.S. Pat. Nos. 4,941,997; 4,973,429; and 5,068,318. Like plasma treatments, these techniques are not cost-effective methods that may easily be incorporated into automated production processes for making biomedical devices such as contact lenses.
A more recent technique developed for coating substrates is a layer-by-layer (“LbL”) polymer absorption process, which is described in “Investigation of New Self-Assembled Multilayer Thin Films Based on Alternately Adsorbed Layers of Polyelectrolytes and Functional Dye Molecules” by Dongsik Yoo, et al. (1996). The process described in this article involves alternatively dipping hydrophilic glass substrates in a polyelectrolyte solution (e.g., polycations such as polyallylamine or polyethyleneimine) and then in an oppositely charged solution to form electrically conducting thin films and light-emitting diodides (LEDs).
Two other similar processes are described in “Molecular-Level Processing of Conjugated Polymers” by Fou & Rubner and Ferreira & Rubner, respectively. These processes involve treating glass substrates that have hydrophilic, hydrophobic, negatively, or positively charged surfaces. The glass surfaces are treated for extended periods in hot acid baths and peroxide/ammonia baths to produce a hydrophilic surface. Hydrophobic surfaces are produced by gas-phase treatment in the presence of 1,1,1,3,3,3-hexamethyldisilazane for 36 hours. Charged surfaces are prepared by covalently anchoring charges onto the surface of the hydrophilic slides. For example, positively charged surfaces are made by further treating the hydrophilic surfaces in methanol, methanol/toluene, and pure toluene rinses, followed by immersion in (N-2 aminoethyl-3-aminopropyl) trimethyloxysilane solution for 12 to 15 hours. This procedure produces glass slides with amine functionalities, which are positively charged at a low pH.
In addition to the above-described techniques, U.S. Pat. Nos. 5,518,767 and 5,536,573 to Rubner et al. describe methods of producing bilayers of p-type doped electrically conductive polycationic polymers and polyanions or water-soluble, non-ionic polymers on glass substrates. These patents describe extensive chemical pre-treatments of glass substrates that are similar to those described in the aforementioned articles.
Various layer-by-layer polyelectrolyte deposition techniques have also been developed by the assignee of the present invention. These layer-by-layer techniques effectively alter the surfaces of various materials, such as contact lenses. One such technique is described in co-pending U.S. Patent Application Ser. No. 60/180,576 filed on Feb. 4, 2000, entitled “Apparatus, Methods, and Compositions for Modifying Surface Characteristics”. In particular, a layer-by-layer technique is described that involves consecutively dipping a substrate into oppositely charged polyionic materials until a coating of a desired thickness is formed.
In addition, another technique that results in a layer-by-layer coating while avoiding the time-consuming aspects of sequential dipping, is the single dip process disclosed in co-pending U.S. Patent Application Ser. No. 60/180,463 filed on Feb. 4, 2000, entitled “Single-Dip Process for Achieving a Layer-by-Layer-Like Coating”, which applies polyionic material onto the substrate with only a single dip. In this technique, a generally hydrophobic article such as a contact lens is dipped into a single polyionic solution containing at least one polycationic material and at least one polyanionic material. The polycationic material may include a positively charged moiety such as poly(allyl amine hydrochloride) and the polyanionic material may include a negatively charged moiety such as polyacrylic acid. Typically, the polyionic components are employed in non-stoichiometric amounts such that one of the components is present within the solution in a greater amount than another component.
Each of these surface modification techniques are effective for producing a substrate with a surface that is different from the remainder of the substrate. It would be particularly desirable if such modified surfaces were capable of adhering various active agents, such as anti-microbial agents, or other substances, such as photo-initiators, organoselenium, etc. to the substrates. In addition, it would be desirable if such substrate surfaces contained reactive sites for attaching agents through classical chemical attachments processes such as precipitations reactions, hydrogen bonding, electrostatic deposition processes, free radical-initiated polymerization reactions, condensation reactions, and the like.